Why are you using this thing? What is it that you expect this thing to tell you? Before you can convert the value returned by analogRead to something useful, it is necessary to know what the sensor is measuring, what the output of the sensor is, whether the output is linear or logarithmic, and what you want to do with the output.

You seem to have omitted a few of these details and not enabled us to find any of them.

Since the datasheet expresses the PPM value as a ratio of the resistance of the device of the device in clean air with 100PPM carbon monoxide, it suggests that the PPM value isn't accurately available without calibration with a sample gas of the correct constituants. Which is what Retrolefty said several posts back. There is a value given for clean air which is much easier to obtain but with a much lesser degree of accuracy. It still needs you to calibrate the thing though. If accuracy isn't too much of an issue, the first step would be to wire the thing up and see what its resistance is when its been running a while in clean air. Then when you have this figure you work out a nice value for a static resistor for the other side of the voltage divider for the anticipated ranges you want it to measure. I'd use a lookup table to get a figure out what the PPM is.

A completely unsuitable device to be selling to home users in other words....

Description says that there are 2 outputs:analog output, and TTL-level outputI do not fully understand what is the usage of TTL-level output:TTL output valid signal is low (low output signal light, which can be accessed microcontroller IO port)

I'm already reading from MQ-135. I want to buy another one which is mounted on a little pcb with a LM393.There is an extra pin that they said it can be used for digital read from a microprocessor.LM393 is low power dual voltage comparator. I would like to know if anyone is using that kind a sensor.